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I need a new electrical panel. One electrical contractor looked at the
existing feed and said that he could pull 150A wiring through the
existing pipe and change the main breaker in the 200A panel to 150A so
that the breaker matched the wiring coming in.

I am just curious . . , what does the "existing pipe" mean? Is it an
overhead service and the existing pipe is the conduit down the side of the
house? Or, is the "existing pipe" and underground conduit to the house?
Also, what is the size of the "existing pipe"?

Part of why I am curious is that, if it is an overhead drop, maybe changing
the conduit/pipe at the same time as the new service line is be installed
wouldn't cost much, and maybe the new service could just be 200A for not
much more money -- mostly just the additional materials cost.

Another contractor insists that this is not legal because someone
could change that 150A main breaker to 200A in the future because the
panel is rated for 200A.

My next door neighbor did this and it passed inspection. But was it a
clueless inspector or is it really legal?

I noticed that later you discovered that your neighbor didn't end up getting
his inspected after all.

In my area, the normal process is for the contractor to pull the permit
(which I pay for) and then the contractor does the work and a final
inspection is done. That way, I know that the work that is done will meet
all of the codes and will have a final approval sticker upon completion.